The present invention relates to graphite nanofiber catalyst systems for use in the production of fuel cell electrodes. The graphite nanofibers are comprised of graphite sheets aligned either substantially perpendicular or substantially parallel to the longitudinal axis of the nanofiber.
Electrochemical fuel cells are devices that can directly convert chemical (fuel) into electrical energy and are about three times more efficient than thermal conversion systems. Of the various types of fuel cells, the proton exchange membrane (PEM) is preferred for transportation (automobiles) and small portable device (telephones, laptop computers) applications because of their lightweight and low temperature operation. They also offer quick startup, they operate instantly at full capacity, and they rapidly adjust to variable power demands. In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of protons from the anode to the cathode. In addition to conducting protons, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode electrocatalyst layer, oxygen reacts with the protons that have crossed the membrane to form water as the reaction product. The anode and cathode reactions in hydrogen/oxygen fuel cells are shown in the following equations:
Anode reaction: H2xe2x86x922H++2exe2x88x92
Cathode reaction: 1/2O2+2H++2exe2x88x92xe2x86x92H2O
In electrochemical fuel cells employing methanol as the fuel supplied to the anode (so-called xe2x80x9cdirect methanolxe2x80x9d fuel cells) and an oxygen-containing oxidant stream, such as air (or substantially pure oxygen) supplied to the cathode, the methanol is oxidized at the anode to produce protons and carbon dioxide. Typically, the methanol is supplied to the anode as an aqueous solution or as a vapor. The protons migrate through the ion exchange membrane from the anode to the cathode, and at the cathode electrocatalyst layer, oxygen reacts with the protons to form water. The anode and cathode reactions in this type of direct methanol fuel cell are shown in the following equations:
Anode reaction: CH3OH+H2Oxe2x86x926H++CO2+6exe2x88x92
Cathode reaction: 3/2O2+6H++6exe2x88x92xe2x86x923H2O
Platinum supported on carbon, typically a high temperature (graphitized) treated carbon, has been the combination of choice as the catalyst system since both platinum and carbon are resistant to oxidation at the operation conditions of a PEM fuel cell. Although there are still problems to be addressed with this technology, one of the major disadvantages is the initial cost of the system with regard to the membrane and the high amount of noble metal catalyst required for the electrodes. A large fraction of the noble metal catalyst is wasted and because of sintering (increase in particle size) and because of metal particle location with respect to the membrane.
When metal catalyst particles are dispersed on conventional carbon materials, even on conventional graphitic materials, they typically exhibit relatively weak interactions with the basal plane regions of the carbon resulting in the formation of relatively large globular entities (like oil and water). Most of the metal atoms are contained in the globular entities and are consequently unavailable to perform the desired catalytic reaction. It would be highly desirable if the catalytic particles could be deposited in such a manner that they were spread in the form of a thin film over the surface of the carbon. The resulting catalyst-containing structure would give rise to the most efficient use of catalytic metal and as a consequence, it would be possible to not only optimize the catalytic efficiency of the electrode system, but it would also be possible to reduce catalyst loading.
In accordance with the present invention, there is provided a fuel-cell electrode comprised of a dispersion of one or more noble metals, alloys or bimetallics thereof, on graphite nanofibers characterized as: a) comprised of graphite sheets that are substantially parallel or perpendicular to the longitudinal axis of the nanofiber; and b) having at least about 95% of their exposed surfaces comprised of edge regions.
In a preferred embodiment of the present invention, the graphite nanofiber is further characterized as possessing: (i) a surface area from about 0.2 to 3,000 m2/g as determined by N2 adsorption at xe2x88x92196xc2x0 C., (ii) a crystallinity from about 50% to about 100%, and (iii) a distance from about 0.335 nm to about 0.67 nm between the graphite sheets.
In a preferred embodiment the noble metal is selected from the group consisting of Pt, Pd, Rh, Re and mixtures thereof.
In yet another preferred embodiment of the present invention, at least a portion of the exposed edges are capped by the substitution of heteroatoms, such as phosphorous and boron oxides.
In another preferred embodiment, at least a portion of the edges of the graphite nanofiber contain a functional group selected from the group consisting of Cxe2x80x94OH, Cxe2x95x90O, Cxe2x80x94Oxe2x80x94C, and COOH.
In still another embodiment of the present invention, the graphite nanofibers are characterized as having a crystallinity greater than about 90%.
In yet other preferred embodiments, the carbon nanofibers are characterized as having: (i) a surface area from about 50 to 800 m2/g; and (ii) a crystallinity from about 95% to 100%.
In still another preferred embodiment of the present invention, there is provided a hydrogenation catalyst comprised of said carbon nanofibers and one or more hydrogenation-active metals, typically those from Groups VI and VIII of the Periodic Table of the Elements.